1. Field of the Invention
The present invention relates to the evaluation of formation fluids produced into a wellbore. More particularly, the present invention relates to a system that combines multiple sensors in a single downhole module for compensated multiphase flow evaluation.
2. Description of the Related Art
In petroleum producing wells it is not uncommon to find the well fluid flow regime consisting of multiple phases, such as oil and water, oil and gas, or oil, water and gas. Often, one or more of these phases is an undesired element in the well production flow. For example, in the case of a well fluid flow regime consisting of oil and water, the oil is typically the fluid phase desired to be produced and the water is typically an undesired phase in the production flow. When the degree of water present in the well production flow becomes excessive, logging surveys are run at a plurality of depth locations within the well to facilitate the determining of the flow rates of the individual phases at each of the locations. From these flow rate determinations, which will yield information regarding the depth locations and rates of water entry, remedial actions to control such water entry may be chosen.
A spinner (or impeller) type flowmeter is typically used to measure flow velocity from which an overall flow rate is determined. The impeller rotates as it is impinged by the downhole flow. As is known in the art, the impeller angular rotation speed (typically in revolutions per second) is related to the product of the fluid density and the fluid velocity, where the fluid velocity is further used to determine flow rate. Each fluid type has a unique slope, also known as a conversion factor, which is related to the fluid density. In addition, due to bearing friction, each type of fluid has a unique velocity required to initiate impeller motion. At any given impeller speed, multiple fluid velocities are possible depending on which conversion factor is used. Therefore, to determine a flow rate from an impeller angular speed requires knowledge of the fluid density or knowledge of the fluid type from which an appropriate conversion factor can be inferred.
The composition of formation fluids can be identified by certain electrical characteristics. Hydrocarbon fluids have a low conductivity, while salt water brines typically found in subsurface formations have a relatively high conductivity. Because of this fundamental difference in conductivity, downhole sensors can be developed and used to measure the conductivity of the formation fluids. Relative conductivity is evaluated by measuring the amount of current transmitted through the formation fluid sample between two or more electrodes when a selected voltage is applied to them.
In addition to conductivity characteristics, most fluids have a specific dielectric permittivity that can be used to identify them. Dielectric permittivity sensors are usually constructed as a capacitor and measure changes in the capacitor""s dielectric.
In some prior art tool combinations, a fluid typing sensor has been located above, below, or to the side of the spinner element. By not sensing the fluid type of the same fluid that is intercepted by the spinning element, errors have been introduced due to the inhomogeneities of the multi-phase fluid. Accordingly, a need exists for an improved downhole system that can accurately and efficiently evaluate the flow of multi-phase formation fluids.
The methods and apparatus of the present invention overcome the foregoing disadvantages of the prior art by providing a spinner type flowmeter with fluid typing sensors mounted in close proximity to the impeller, thereby providing a more reliable system for determining the flow rate of multi-phase fluids.
The present invention contemplates a compensated flow measuring system for measuring a multi-phase fluid flow in a well. The flow measuring system uses a combination of a spinner flowmeter and a fluid typing sensor system in a single measuring device. The close proximity of the fluid typing system to the rotating elements of the flowmeter ensures that the correct fluid type flow conversion factors are utilized for measuring a multi-phase fluid flow.
In a preferred embodiment, the compensated flow measuring system comprises an impeller assembly for intercepting a downhole multi-phase flow and generating an electrical signal related to the indicated flow; a fluid typing system using a conductivity sensor and a capacitance sensor, in close proximity to the impeller, for determining the electrical admittance of the flowing fluid and generating an electric signal related to the fluid type; and, an electronics system for powering the impeller assembly and the fluid typing system and for receiving and analyzing the signals from the impeller assembly and the fluid typing system and outputting a signal related to a compensated flow rate of the multi-phase fluid.
In one preferred embodiment, the conductivity sensor and the capacitance sensor are combined in a single probe which is mounted on an impeller cage arm. The arm acts as an alternating current transmitter for generating conductive and displacement currents into the fluid. The transmitted signal is at a preferred frequency of about 66 kHz. The currents are sensed by the combined sensors in the single probe. The currents are converted into voltage signals. A phase detector separates the combined conductive and capacitance signals and outputs separate voltages related to the conductive and capacitance currents. A processor analyzes the output voltages and acts according to programmed instructions to generate a signal related to a compensated flow rate.
In another preferred embodiment, the transmitted frequency is in the range from about 40 kHz up to and including about 200 kHz.
The method of the invention is practiced by intercepting a multi-phase flow in a well with an impeller assembly having said impeller assembly output an electrical signal related to the flow rate; determining an electrical characteristic of the fluid, and analyzing the fluid flow and the electrical characteristic to generate a signal related to a compensated multi-phase flow rate.
Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.